1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which excite nuclear spin of an object magnetically with an RF (radio frequency) signal having the Larmor frequency and reconstruct an image based on an MR (magnetic resonance) signal generated due to the excitation and, more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which acquire a magnetic resonance image by using SSFP (Steady-State Free Precession).
2. Description of the Related Art
Magnetic Resonance Imaging (MRI) is an imaging method which excites nuclear spin of an object set in a static magnetic field with an RF signal having the Larmor frequency magnetically and reconstructs an image based on an MR signal generated due to the excitation. A magnetic resonance imaging apparatus operates according to a pulse sequence to define an imaging condition. In a recent magnetic resonance imaging apparatus, a pulse sequence using phenomenon called SSFP (SSFP sequence) for cardiac cine imaging and coronary imaging is often used.
FIG. 1 is a diagram showing a conventional SSFP sequence.
In FIG. 1, RF denotes RF signals to be transmitted to an object, Gss denotes slice gradient magnetic field pulses to be applied to an object for slice selection, Gro denotes RO (readout) gradient magnetic field pulses to be applied to an object for readout of echo data from the object (also referred to as frequency encode gradient magnetic field pulses), Gpe denotes PE (phase encode) gradient magnetic field pulses to be applied to an object for phase encode.
An SSFP sequence is a sequence to acquire echo data with making a magnetization spins in a static magnetic field into a steady state by repeating an RF excitation. Specifically, as shown in FIG. 1, in the SSFP sequence, subsequently to an application of a +α/2 excitation pulse or a −α/2 excitation pulse for start up, a +α excitation pulse (flip pulse) and a −α excitation pulse are applied repeatedly in mutual with a constant repetition time (TR). Then, a magnetized spin in a static magnetic field is maintained on a steady state by applying the +α excitation pulse and the −α excitation pulse. Generally, each of the ±α excitation pulses is applied with a slice gradient magnetic field pulse Gss for a selective excitation of a desired slice. An RO gradient magnetic field pulse Gro and a PE gradient magnetic field pulse Gpe are also applied for adding space information. Note that, it is not necessary that the +α/2 excitation pulse or the −α/2 excitation pulse for start up is applied.
Each integration value of a slice gradient magnetic field pulse Gss, an RO gradient magnetic field pulse Gro and a PE gradient magnetic field pulse Gpe in a TR is controlled to be zero. An applied phase of an excitation pulse is controlled to shift linearly by a constant angle. Generally, a constant angle is set to be 180-degrees for an on-resonance spin.
Since the SSFP sequence as mentioned above doesn't spoil a part of signal, obtaining an image with relatively high S/N (signal-to-noise) ratio rapidly is a great advantage. While an SSFP sequence can obtain a constant image contrast by maintaining a steady state, the demand for obtaining different image contrast without lack of the merit above mentioned is increasing.
An SSFP image with a fat saturation is an example demanded for different image contrast. It is known that fat has strong signal in the steady-state since a fat has relatively short longitudinal relaxation (T1) time. Therefore, when an abdominal image is obtained by an SSFP sequence, there is a case that it is difficult to depict anatomical structure and lesions due to strong fat signal. Consequently, fat suppression is required to obtain an abdominal image by an SSFP sequence.
To the contrary, a fat suppression technique is devised by setting the interval called preparation block in an SSFP sequence and applying a pre-pulse for fat saturation in the preparation block (see, for example, Scheffler et al., Magnetic Resonance in Medicine, Vol. 45, pages 1075-1080 (2001)).
FIG. 2 is a diagram showing a conventional SSFP sequence with a preparation block.
FIG. 2 shows RF signals applied to an object in time series. As shown in FIG. 2, in an SSFP sequence with a preparation block, subsequently to an application of a −α/2 excitation pulse for start up, an excitation pulse train with a certain excitation angle ±α is applied. Further, on the excitation pulse train, a +α/2 flip back pulse is applied. After applying the +α/2 flip back pulse, the interval called preparation block is set. Subsequently to the preparation block, a +α/2 start up pulse is applied. Then, after applying the +α/2 start up pulse, an application of the α excitation pulse train with a certain excitation angle ±α is restarted.
In a preparation block, so called RF pre-pulse such as a fat saturation pulse is applied for suppressing a fat signal. The character of this method is to set a preparation block under a state in which a spin is stored as a longitudinal magnetization by an α/2 flip back pulse. In a preparation block, generally, a spoiler gradient magnetic field pulse for spoiling a transverse magnetization is applied in addition to an RF pre-pulse. As described above, in an SSFP sequence, a preparation block is set for varying an image contrast.
Further, conventionally, an SSFP sequence is also used for imaging by an inversion recovery (IR) method. IR is an imaging method that applies a 180-degree IR pulse to obtain an image with signal intensity depending on recovery due to T1 from a state in which a spin is inverted.
FIG. 3 is a diagram showing a conventional SSFP sequence with an IR pulse.
In FIG. 3, RF denotes RF signals to be transmitted to an object, Gss denotes slice gradient magnetic field pulses, Gro denotes RO gradient magnetic field pulses, Gpe denotes PE gradient magnetic field pulse.
When an IR pulse is applied in an SSFP sequence, as shown in FIG. 3, application of the α excitation pulses applied continuously with the TR is stopped once. Then, after a spoiler pulse is applied subsequently to the IR pulse, application of continuous excitation pulses is restarted again. Note that, a start up pulse or a flip back pulse is occasionally used together.
In addition, a GCFP (global coherent free precession) method is proposed as an applicable tagging technology in an SSFP sequence. The GCFP method is a technology which applies coherent spin labeling. Specifically, the GCFP method is a technique to catch only a proton, passing an MRI scan cross-section, of water molecule in blood cells with a radio frequency wave when the proton passes the MRI scan cross-section, i.e., tagging technology.
In a conventional SSFP sequence with a preparation block, the T1 relaxation of a spin progresses in the period from the α/2 flip back pulse to the α/2 start up pulse. In addition, in a preparation block, phase continuity of a spin is destroyed by applying an RF pre-pulse or by applying a spoiler gradient magnetic field pulse. It is reported that effect due to progression of T1 relaxation of a spin and destruction of phase continuity of a spin is a little in water component showing a long T1. However, there is a problem that effect due to progression of T1 relaxation of a spin and destruction of phase continuity of a spin is not negligible on tissues each having a short T1 in a living body.
That is, on the process of an SSFP sequence, T1 relaxation occurs due to applying an α/2 flip back pulse and an α/2 start up pulse and by setting a preparation block, and therefore, a blocking of phase continuity occurs due to the preparation block. T1 relaxation and the blocking of phase continuity are factors to vary an image contrast obtained by an SSFP sequence. Accordingly, there is a possibility that an image artifact appears due to a shift from an SSFP state.
Therefore, a method that keeping phase continuity of a spin in an SSFP sequence and varying an image contrast by an RF pre-pulse can be achieved at the same time is required.
In a conventional SSFP sequence with an IR pulse, since an application of an α excitation pulse stops in the middle, the first spin after an application of an excitation pulse is restarted is in a state different from a steady state. This result caused contrast variation of an image and appearance of an artifact.
In addition, a slice gradient magnetic field and an RO gradient magnetic field are common and fixed in the GCFP method. Further, the GCFP method has a disadvantage that a radial acquisition cannot be performed since applying direction of a PE gradient magnetic field pulse is limited in the direction perpendicular to a slice.